Fuel cell by-product

[Tyko]

I believe there are some solar power plants today that use surplus daytime energy to electrolyze water for Hydrogen and then burn the hydrogen at night to continue supplying 'round-the-clock power. So, this would work in theory. I think the efficiencies of each step would make or break the idea. converting fuel to electricity involves some efficiency loss, converting the byproduct back into fuel would involve another efficiency loss.

Edited December 8, 2016 by Tyko

[Buzz light fear]

20 hours ago, EpicSpaceTroll139 said:

I can see a fuel cell byproduct a being very useful. Perhaps there could be too fuel cell types: 

Total loss: What we have now.

Reversible/Closed Cycle: More expensive and heavy than an equivalent total loss fuel cell. Produces "water" (or whatever reacted LfOx forms) as a byproduct, of which some can be stored in a tank integrated in the fuel cell, and if necessary, some can be stored in separate tanks. When toggled, the fuel cell would act as an electrolyzer, using somewhat more electricity than was produced earlier in order to revert the byproduct into LfOx. Such a system could be useful for dense energy storage to say, allow power hungry operations (such as a running a MPL) to go on through, for example, the long Münar night. As opposed to having a few hundred large batteries, you could have a few fuel tanks and a reversible fuel cell. The trade-off would be that the maximum charge/discharge rates would be finite, as opposed to infinite like with regular KSP batteries.

Yes that is a good idea

[Magzimum]

1 hour ago, Tyko said:

I believe there are some solar power plants today that use surplus daytime energy to electrolyze water for Hydrogen and then burn the hydrogen at night to continue supplying 'round-the-clock power. So, this would work in theory. I think the efficiencies of each step would make or break the idea. converting fuel to electricity involves some efficiency loss, converting the byproduct back into fuel would involve another efficiency loss.

According to this article (wikipedia), a fuel cell is typically 40-60% efficient in turning its chemical energy into electricity. And according to this article (also wiki), the large-scale electrolysis plants can convert water into hydrogen and oxygen at 65-70%. So, the losses are significant. Using these numbers, the best case scenario you have only 42% of your useful energy left after one cycle (since energy is conserved, and since thermodynamics has no humor, all the other 58% is converted to waste heat, which you can use to warm your hands if you are lucky, but not to run any equipment).

I never looked up these efficiency numbers, and I was quite shocked to read how low it was, but other wiki articles agree, although they specify that this is the AC to AC round trip efficiency... It appears that the main feature of hydrogen storage is the storage density: i.e. a tank full of hydrogen represents a lot of energy - especially if you don't have to carry around the oxidizer, like on Earth. That light-weight makes it interesting for mobile applications, such as cars. Pumped-storage hydroelectricity ( = pumping water up into an artificial lake, then using hydro-power to turn it back to electricity) has an overall efficiency of around 70%-80% which is a LOT better already. Batteries are probably the best in terms of efficiency (at least of the practical real-world options)? 

Anyway, little more background on our electricity-to-chemical problem here: There are several schemes where excess electricity is turned into a chemical product. As far as I know, the basis is always splitting water into hydrogen and oxygen, but in several such processes, also CO₂ is used, so that a carbon atom can be used to create some new chemicals such as methane and methanol. (The hydrogen is reacted with CO2, forming methane, so the core of that processes has the same efficiency as described above). The hydrogen and methane production processes are at or near commercial scale (which says everything about the state of the technology, and nothing about their profitability). 

Whether any of these methods to store excess electric energy in a chemical storage would be efficient for space depends, next to their efficiency, on the mass of a storage tank vs the mass of a battery. Batteries are expensive to make, but relatively light weight. So, for us surface-bound terrans, building a large tank to store hydrogen is a lot cheaper than building a battery. But for Kerbals in LKO or on some Mun base, the low mass of the batteries may be a really good selling point. (Note that in space, we also have to count the mass of the ISRU unit itself, and the oxidizer storage, both of which are not counted when we discuss the hydrogen cars on Earth).